EP0843837A1

EP0843837A1 - Fibre optic device for homogenising a laser beam

Info

Publication number: EP0843837A1
Application number: EP96927747A
Authority: EP
Inventors: Bruno Godard; Robert Stehle
Original assignee: Societe de Commercialisation des Produits de la Recherche Appliquee SOCPRA; Production Et De Recherches Appliquees Ste
Current assignee: Societe de Commercialisation des Produits de la Recherche Appliquee SOCPRA; Production Et De Recherches Appliquees Ste
Priority date: 1995-08-11
Filing date: 1996-08-06
Publication date: 1998-05-27
Anticipated expiration: 2016-08-06
Also published as: FR2737786A1; KR19990036208A; FR2737786B1; CA2224997A1; US6014260A; KR100404297B1; EP0843837B1; JP2000502191A; ATE181425T1; DE69602952T2; DE69602951T2; KR100404298B1; EP0843836B1; DE69602952D1; KR19990036209A; CA2224998A1; US6055346A; JP2000502190A; CA2224998C; EP0843836A1

Abstract

PCT No. PCT/FR96/01251 Sec. 371 Date Jan. 21, 1998 Sec. 102(e) Date Jan. 21, 1998 PCT Filed Aug. 6, 1996 PCT Pub. No. WO97/07423 PCT Pub. Date Feb. 27, 1997An optical device for homogenizing a laser beam is described. Such a device is commonly used in the surface treatment of an object. The device consists of front lenses that are bonded together to form an assembly of lenses that breaks the laser light into multiple beams. This results in the homogenization of the gaussian profile of the beam intensity as viewed perpendicular to the beam's propagation. Advantageously the matrix of lenses is formed using groves or bevels in a manner that minimizes losses resulting from light impinging on glue disposed upon the entire lateral surface of the front lenses. In the invention gaps between front lenses that cause interference with the light path are eliminated. This gap free bonding is accomplished by filling the groves or bevels with glue, inserting a rod or wire into the grove to assemble the front lenses in a support frame, bonding a grill to assembled front lenses, or by using a combination of gluing with a support frame.

Description

Fiber optic device for homogenizing a laser beam

The present invention relates to a fiber optic device for homogenizing a laser beam.

It finds general application in any technical field using a laser, and in particular in laser surface treatment, such as annealing of amorphous silicon, cleaning, polishing and surface preparation, in particular.

In general, the energy distribution along the cross section of a laser beam, at least in the lowest emission order, is of Gaussian appearance, in any case variable according to the cross section of the beam.

However, to obtain, for example, an optimal annealing of amorphous silicon, it is necessary to apply a laser beam having a homogeneous and substantially uniform energy distribution.

This homogeneity is especially required when the sample to be treated by laser is large (as described in the French Patent Application entitled "Device and method of surface treatment by laser", filed by the Applicant on August 11 1995 under No. 95 09778., the content of which forms an integral part of this Request, for all practical purposes). It is also required when the incident laser beam is a combination of several laser beams emanating from a chain of laser modules arranged in parallel and / or in series (as described in the Patent Application entitled "Method and device for controlling 'a laser source with several laser modules to optimize surface treatment by laser ", filed by the Applicant on August 11, 1995 under No. 95 09780, the content of which forms an integral part of this Application, for all practical purposes) . Means are already known for homogenizing a laser beam.

However, these means are, most often, complicated, expensive, difficult to implement and ill-suited to large beams, and high power.

The present invention provides a solution to this problem.

It relates to a fiber optic device for homogenizing a laser beam, of the type comprising:

- A plurality of front lenses, contiguous, arranged in m rows and n columns, perpendicular to the direction of propagation of the laser beam to be treated, and suitable for cutting said laser beam into mxn laser beams each having a substantially uniform cross section, and a substantially homogeneous energy distribution, and

- At least one collection lens, converging, arranged perpendicular to the direction of propagation of the light beam, downstream of the front lenses in the direction of the path of the laser beam, said collection lens being able to focus the beams in a chosen plane from the front lenses.

According to a general definition of the invention, the device further comprises:

- A first intermediate plane, substantially located in the focal plane of the front lenses, and having a plurality of entrance pupils each arranged substantially at the focal point of a front lens chosen from the plurality of front lenses;

a second intermediate plane having a plurality of exit pupils, the collection lens being able to collecting the beams from the plurality of exit pupils; and

- Optical transmission means capable of individually transmitting the light beams coming from the entrance pupils to the plurality of exit pupils.

The transmission means are thus able to transfer a plurality of homogeneous portions of the laser energy. In the case of a high power laser, the device according to the invention thus makes it possible to divide and transfer laser energy into a plurality of homogeneous portions of low power energy. This results in the elimination of the damage that can be caused by the effects of a high laser power, in particular on the optical elements of the device according to the invention.

Such a device also makes it possible to obtain a homogeneous beam in the near field.

It finds an advantageous application when the beam to be homogenized is a combination of several laser beams emanating from a chain of laser modules placed in parallel and / or in series.

According to a preferred embodiment of the invention, the optical transmission means comprise a plurality of optical fibers each comprising one end coupled to an entrance pupil and another end coupled to an exit pupil.

For example, optical fibers have a diameter of around 125 μm and a length of less than 1 m. As a variant, they have a length greater than 1 m.

According to another aspect of the invention, the plurality of optical fibers of indices i, j transmitting the laser beams coming from the front lenses are capable of distributing them according to a distribution of indices k, S. where at least some of the indices k and JE! are different from the indices i and j. As a variant, the indices i and j are respectively equal to the indices k and £.

According to another embodiment of the invention, the device further comprises a third intermediate plane having a plurality of additional front lenses, contiguous, arranged in q rows and r columns, perpen¬ dicular to the direction of propagation of the beams laser from the plurality of exit pupils, and adapted to receive said laser beams from the plurality of exit pupils as well as to cut them each into qr laser beams each having a substantially uniform cross section, and a substantially homogeneous energy distribution ; the collection lens being suitable for collecting in a chosen plane the laser beams coming from the additional front lenses.

According to another embodiment of the invention, the optical transmission means comprise a plurality of discrete optical elements.

Other advantages and characteristics of the invention will become apparent in the light of the detailed description below and of the drawings in which:

- Figure 1 is a schematic representation of the essential means of the homogenizer;

- Figure 2 is a variant of the homogenizer;

- Figure 3 is a homogenizer with two collection lenses;

- Figure 4 illustrates an assembly of the front lenses of the homogenizer;

- Figure 5 schematically shows the plurality of optical fibers receiving the laser beams cut by the front lenses for routing them to the collection lens according to the invention;

- Figure 6 shows schematically ia distribution of laser beams cut by the front lenses according to a crossed distribution of the optical fibers according to the invention;

- Figure 7 shows a variant of the device described with reference to Figure 5 in which the laser beams cut by the front lenses and distributed by the optical fibers are cut again by a plurality of additional front lenses according to the invention; and

- Figure 8 is a variant of the device described with reference to Figure 6 in which the laser beams cut by the front lenses and distributed by the optical fibers are cut again by a plurality of additional front lenses.

In FIG. 1, the reference FLA designates the laser beam to be homogenized. This laser beam has a non uniform energy distribution in cross section. It emanates for example from a laser or from a chain of laser modules placed in parallel and / or in series.

For example, this laser beam is intended for annealing amorphous silicon by laser. One application is to illuminate a large panel of amorphous silicon for the manufacture of flat liquid crystal screens. This laser beam is applied to the target plane CIB carrying the sample to be treated.

The optical device for homogenizing a laser beam comprises mxn front lenses LF, convergent, jointed, arranged in m lines and n columns perpendicular (transversely) to the direction of propagation of the laser beam to be treated, m and n are whole numbers, for example m and n are equal to 7. The front lenses are arranged according to a straight or oblique rectangular matrix; for example, they are regularly distributed within a rectangle or a parallelogram.

These lenses cut the FLA laser beam into m × n laser beams, each having a substantially uniform cross section and a substantially homogeneous energy distribution.

There is provided a collection lens LC, convergent, arranged perpendicular to the direction of propagation of the light beam, downstream of the front lenses LF in the direction of travel of the laser beam. This collection lens is suitable for focusing, in the target plane CIB, the beams coming from the front lenses.

m x n DI diaphragms can be associated respectively with m x n front lenses. Each diaphragm is arranged substantially in the object focal plane of the associated lens and receives the light beam from the associated front lens in order to filter it spatially.

The reference "a" designates the width of a front lens LF. The reference f designates the focal distance (close to the print) between a front lens and the associated diaphragm DI. The reference F designates the distance (which may be different from the focal distance) between a DI diaphragm and the collection lens LC. The reference A designates the width of the homogenized laser beam obtained by the homogenizer HO at the level of the plane CIB according to the invention. The dimension A is equal to a x F / f.

Each DI diaphragm comprises an opening of selected shape and dimensions, for example circular. A support or mask MA maintains the plurality of diaphragms.

The front lenses are for example of the convex-plane or biconvex, or convex-concave type. It is made so that the cross section of the light beam coming from each front lens whose shape is determined by the shape of the lenses, is rectangular or hexagonal, downstream of the diaphragms.

When the LC collection lens is small in diameter, the optical device is less expensive and has fewer optical aberrations than with a large collection lens.

In Figure 2, the LC collection lens is arranged so that the focal plane of the LF lenses is not confused with that of the LC collection lens. This variant gives a reduction in the size of the homogenizer device.

The path of the light rays shows here that the homogenizing device is not necessarily afocal.

With reference to FIG. 3, the collection lens LC is replaced by two converging lenses LC1 and LC2, arranged perpendicular to the direction of propagation of the light beam, downstream of the front lenses LF and the associated diaphragms DI.

Very advantageously, these collection lenses LC1 and LC2 are movable in translation along the optical axis.

The distance d between the two lenses LC1 and LC2 is varied to vary the resulting focal length and to obtain, at the level of the target plane CIB, a homogenized beam size suitable for the chosen application, for example the size of the panels to be treated in the case of an amorphous silicon annealing.

It should be noted that the other optical elements, namely the front lenses LF, the diaphragms DI and the target plane CIB can also be movable in translation along the optical axis. The choice of the relative distances between the different optical elements makes it possible not only to adapt the size of the homogenized laser beam to the chosen application (possibility of choosing the magnification by varying the distance d), but also to reduce the bulk. of the homogenizer device.

It should also be noted that the choice of the distance d also makes it possible to adjust the energy per unit area delivered by the laser beam, as described in the Patent Application filed by the Applicant for "Method and control device a laser source with several laser modules to optimize the surface treatment by laser "and already mentioned above.

In FIG. 4, the path of several beams is shown through several front lenses LF of the same line m. To facilitate understanding of the invention, only three front lenses are shown diagrammatically next to each other. These front lenses are for example of the convex-plane type. They are generally rectangular in shape with an inlet face FE, an outlet face FS and four lateral faces FL1 to FL4. The convex part receives the laser beam to be treated FLA. A lateral face of a front lens is defined for example by the points A2, A'3, B'3 and B2 for the lateral face FL3 of the central lens LF2.

The laser beam to be treated from point A1 is applied to the central lens LF2 at point A2. This beam passes through the front lens LF2 to point A3 and is then routed to point A4. It should be noted that point A3 is slightly offset from point A'3.

The convex part of a front lens here comprises two sharp edges such as BV. The front lens LF2 comprises a groove EN4 of shallow depth formed transversely in the lateral face FL1.

Likewise, the front lens LF2 comprises a groove EN3 of shallow depth formed transversely in the lateral face FL2.

Preferably, the grooves are formed near the exit face of the front lens.

The same is true for the other front lenses.

_**

The two adjacent front lenses LF2 and LF3 are attached to each other. The cooperation of the respective grooves EN3 and EN2 allows bonding of said lenses by applying glue in said grooves EN3 and EN2, without disturbing the optical properties of the light beams passing through said front lenses LF2 and LF3.

The path of the beam B1 through the central front lens LF2 passes along the points B2, B3 and B4.

It should be noted that the light path is not disturbed by the notches or grooves EN and the chamfers CH.

Other means of assembling the lenses are possible according to the International Patent Application entitled "Optical device for homogenizing a laser beam", filed by the Applicant on the same day as this Application, and the content of which is an integral part of this Request, for all intents and purposes.

The front lenses are generally rectangular or hexagonal in shape with sides of the order of 1 cm in length.

For example, the beam to homogenize FLA is rectangular with sides of the order of 5 to 8 cm. According to the invention, the laser beams cut by the front lenses are transferred and distributed to the collection lens using optical transmission means such as optical fibers or discrete optical elements.

With reference to FIG. 5, the optical transmission means comprise a plurality of optical fibers FO of indices i and j, capable of individually transferring the laser beams coming from a plurality of entry pupils PE of indices m and n associated with the plurality of front lenses LF of indices m and n to a plurality of exit pupils PS of indices k and £.

By indices m and n or k and £ is meant here a two-dimensional matrix arranged in m columns (or k) and n rows (or £). The indices i and j vary here respectively from 1 to m and from 1 to n.

The plurality of entry pupils PE is located on an intermediate plane PII disposed in the focal plane of the front lenses LF.

The plurality of exit pupils PS with indices k and £ is arranged on an intermediate plane PI2 which can be separate from or superimposed on the intermediate plane PII.

It should be noted that the entrance and / or exit pupils can be materialized or not. Materialization improves spatial filtering. Furthermore, the optical correspondence between the entrance pupils and the exit pupils is here advantageously one-to-one.

It should be noted here that each optical fiber FO contributes for its part to the transfer of a portion of the laser energy. In the case of a high power laser, the device according to the invention thus makes it possible to divide and transfer a high power laser energy into a plurality of lower power portions of energy. This results in the elimination of the damage that can be caused by the effects of high laser power, in particular on the optical elements of the device.

It may be advantageous for these optical fibers FO to be arranged in an at least partially crossed bundle. That is to say that at least some of the fibers of indices i, j each connect an exit pupil PS not having the same values of i and / or j. In principle, these crossings are arranged so that any exit pupil remains supplied from one of the entry pupils.

In other words, the bundle of fibers collecting the energies of the lenses LF _ij can distribute them according to a distribution PS _k _ _g where at least some of the k and £ are different from the i and j. This allows the energy distribution at the output to be brought even closer to the characteristics desired for the latter (FIG. 6).

It should be noted that the optical fibers also make it possible to illuminate a target plane CIB situated outside the optical axis of the laser beams FLA.

In this regard, it is a question in the above of homogé¬ neiser the laser beam obtained at output. Although important applications involve obtaining a beam whose energy density per unit area is uniform in cross section, it is clear that the word "homogenize" does not necessarily imply equality of density d energy over the entire cross section, but on the contrary extends to obtaining any desired distribution of this energy density.

As a variant, it may be necessary to cut the laser beams once more in order to further improve its homogeneity. Under these conditions (FIG. 7), the device according to the invention further comprises an intermediate plane PI3 having a plurality of front lenses additional res LFS of indices q and r, arranged in q lines and r colonies, perpendicular to the direction of propagation of the laser beams coming from the plurality of exit pupils PS _k _g. This plurality of additional front lenses receives the laser beams from the plurality of exit pupils PS _{ki in} order to cut them each into qr laser beams each having a substantially uniform cross section and a substantially homogeneous energy distribution.

The collection lens (s) collect (s) in the chosen plane CIB the laser beams thus cut by the additional front lenses.

the laser beams coming from the exit pupils PS _ki are cut by the additional front lenses LFS _σr according to a distribution where at least some of the indices k and £ are different from the indices q and r.

Advantageously, an intermediate plane PI4 has a plurality of diaphragms DI with indices q and r. The diaphragms DI with indices q and r receive the laser beams with indices q and r from the plurality of additional front lenses LFS in order to filter them spatially.

Optical fibers can be long or short. For example, they have a diameter of the order of 125 μm (core of the fiber).

For a 125 μm core, a short fiber is less than 1 meter long. It retains the shape of the cross section of the beam which penetrates it.

For a 125 μm core, a long fiber is longer than 1 m. It depolarizes the light beam which penetrates it and emits a light beam provided with a symmetry of revolution.

With reference to FIG. 8, the additional front lenses LFS are arranged at a distance 2f from the plane intermediate PI2 and intermediate plan PI4. Under these conditions, the light beams coming from the exit pupils PS illuminate several additional front lenses.

Claims

claims

1. Optical device for homogenizing a laser beam, comprising:

- a plurality of front lenses (LF-), contiguous, arranged in m lines and n columns, perpendicular to the direction of propagation of the laser beam, and suitable for cutting the laser beam to be treated into mxn laser beams each having a substantially cross section uniform, and a substantially homogeneous energy distribution,

- at least one collection lens (LC), converging, arranged perpendicular to the direction of propagation of the light beam, downstream of the front lenses (LF ^) in the direction of the path of the laser beam, said collection lens (LC) being suitable for collecting in a chosen plane (CIB) the beams coming from the front lenses in order to obtain a homogeneous beam,

characterized in that it further comprises

- a first intermediate plane (PII), substantially situated in the focal plane of the front lenses (LF ^), and having a plurality of entrance pupils (PE _{i; j} ) each disposed substantially at the focal point of a front lens (LF ^) selected from the plurality of front lenses (LF ^);

- a second intermediate plane (PI2) having a plurality of exit pupils (PS _{i: j} ), the collection lens being able to collect the beams coming from the plurality of exit pupils (PS _{i: j} ); and

- Optical transmission means capable of individually transmitting the light beams coming from the plurality of entrance pupils (PE _{i: j} ) to the plurality of exit pupils (PS _ki ).

2. Device according to claim 1, characterized in that the optical transmission means comprise a plurality of optical fibers (FO _{i: j} ) each comprising one end coupled to an entrance pupil (PE _{± j} ) and one end coupled to an exit pupil (PS _ki ).

3. Device according to claim 2, characterized in that the optical fibers (FO ^) have a diameter of about 125 microns and a length less than 1 m.

4. Device according to ia claim 2, characterized in that the optical fibers (FO _{i; j} ) have a diameter of about 125 microns and a length greater than 1 m.

5. Device according to claim 2, characterized in that the plurality of optical fibers (FO _{± j} ) transmitting the laser beams from the front lenses (LF _{± j} ) are ^• able to distribute them according to a distribution (PS _KjB ) where some at least indices k and i are different from indices i and j.

6. Device according to claim 2, characterized in that the plurality of optical fibers (FO _{i: i} ) transmit the laser beams coming from the front lenses (LF _j ^) are able to distribute them according to a distribution (PS _ki ) where the indices k and £ are identical to indices i and j.

7. Device according to any one of the preceding claims, characterized in that it further comprises a third intermediate plane (PI3) having a plurality of additional front lenses (LFS _gr ) contiguous, arranged in q rows and r columns, perpendicularly to the direction of propagation of the laser beams coming from the plurality of exit pupils (PS _kjg ), adapted to receive said laser beams coming from the plurality of exit pupils (PS _kje ), as well as to cut them each into qr beams laser each having a substantially uniform cross section, and a substantially homogeneous energy distribution; the collection lens being suitable for collecting in a chosen plane the beams coming from the additional front lenses (LFS _qr ).

8. Device according to claim 7, characterized in that the plurality of additional front lenses

(LFS _qr ) cutting the laser beams coming from the exit pupils (PS _kjg ) are able to cut them according to a distribution (R _σr ) where at least some of the indices k and i are different from the indices q and r.

9. Device according to claim 7, characterized in that it further comprises a fourth intermediate plane (PI4) having a plurality of diaphragms (DI _qr ) adapted to receive the qr laser beams from the plurality of additional front lenses in order to filter them spatially.

10. Device according to claim 1, characterized in that the first and second intermediate planes (PII and PI2) are separate or superposed.

11. Device according to claim 1, characterized in that the optical transmission means comprise a plurality of discrete optical elements.

12. Device according to any one of the preceding claims, characterized in that the front lenses are assembled to one another without disturbing the optical path of the light beams which pass through them.

13. Device according to any one of the preceding claims, characterized in that it comprises two collection lenses (LC1, LC2) movable in translation along the optical axis of the device to vary the resulting focal length.